Plasma membrane (p.m.) isolated from rat sperm cells after incubation with serum albumin was recently demonstrated, in this laboratory, to have an altered lipid composition. Lipid exchange between the protein and p.m. was shown to cause this change, which decreased the cholesterol/phospholipid (c/p) ratio. In contrast, seminal plasma membrane vesicles with decapacitation factor activity (DF vesicles) and liposomes containig cholesterol add sterol to the spermatozoa and also impede albumin-induced sperm capacitation. It is the primary objective of the proposed investigation to further elucidate the molecular basis of sperm capacitation and decapacitation. Initially, an endeavor will be made to establish the effect of the vesicles on lipid make-up in rat sperm p.m. This study would involve the first detailed characterization of sperm p.m. lipids in relation to capacitation. It would include changes in c/p and fluidity. Thin layer chromatography, gas liquid chromatography, ESR and 2D SDS polyacrylamide gel electrophoresis. An endeavor will be made to identify capacitation factors in rabbit uterine fluid. Fluid flushed from the uterus of does in different endocrine states would be fractionated chromatographically and electrophoretically. Isolated fractions shall be assayed for their effect on fertilization in vitro by rat sperm. The fate of DF vesicles in utero will be examined in rabbits. In addition to the significance of the study for fertility control and certain kinds of infertility, it should help clarify how mammalian sperm gain fertilizing ability.